The invention relates to multiphase electrical overload switching relays for protecting a load from overcurrent conditions in one or all of the phases.
Overload relays and switches are known in the art, for example as shown in Woodger U.S. Pat. No. 3,800,260, Fryer U.S. Pat. No. 4,096,465 and Forsell et al U.S. Pat. Nos. 4,520,244 and 4,528,539, incorporated herein by reference.
The present invention and the prior art provides cut-out switching for a three phase overcurrent condition mode and for a loss of phase overcurrent condition mode. The present invention further includes improvements providing a constant ratio relationship of the above modes throughout all ranges of current settings by means of adaptive compensation.
The relay trips in response to a mean value of currents in all phases exceeding a first threshold. The relay also trips in response to loss of current in one of the phases when current in another phase exceeds a second threshold. The first threshold is greater than the second threshold. The invention includes an ambient compensator adjusting both thresholds, affording ambient compensation of the mean value of currents in all phases and affording ambient compensation of single phase loss. Current responsive deflectors, e.g. bimetals, drive transfer actuator structure which moves a first travel distance corresponding to the first threshold and a second travel distance corresponding to the second threshold. The ambient compensator adjusts the length of the first travel distance to adjust the first threshold, and also adjusts the length of the second travel distance to adjust the second threshold. The ratio of the second travel distance to the first travel distance is constant notwithstanding adjustment by the ambient compensator changing the lengths of the first and second travel distances.